Flanked by schoolchildren and reading from their letters, President Obama announced measures he plans to take to prevent gun violence on Wednesday. Also on Wednesday, the NRA began an ad campaign against gun control.

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WASHINGTON — President Barack Obama launched the biggest U.S. gun-control push in generations on Wednesday, urging Congress to approve an assault weapons ban and background checks for all gunbuyers to prevent mass shootings like the Newtown school massacre.

Rolling out a wide-ranging plan for executive and legislative action to curb gun violence, Obama set up a fierce clash with the powerful U.S. gun lobby and its supporters in Congress, who will resist what they see as an encroachment on constitutionally protected gun rights.

Obama presented his agenda at a White House event in front of an audience that included relatives of some of the 20 first-graders who were killed along with six adults by a gunman on Dec. 14 at Sandy Hook Elementary School in Newtown, Connecticut.

In a sign of how bitter the fight over gun control could get, the National Rifle Association released an advertisement hours before Obama spoke that accused him of hypocrisy for accepting armed Secret Service protection for his daughters. The White House condemned the ad as "repugnant."

Until now, Obama had done little to change America's gun culture. But just days before his second inauguration, he appears determined to champion gun control in his next term, which also will be dominated by debt and spending fights with Congress and a likely debate over immigration reform.

His plan calls on Congress to renew a prohibition on assault weapons sales that expired in 2004, require criminal background checks on all gun purchases, including closing a loophole for gun show sales, and pass a new federal gun trafficking law - long sought by big-city mayors to keep out-of-state guns off their streets.

He also announced 23 steps he intends to take immediately without congressional approval. These include improving the existing system for background checks, lifting the ban on federal research on gun violence, putting more counselors and "resource officers" in schools and better access to mental health services.

Obama signed three of the measures at the ceremony.

Assault weapons battle

Obama, who has called the day of the Newtown massacre the worst of his presidency, looked down into the audience and addressed the parents of one of the Sandy Hook victims, Grace McDonald, 7, saying he had hung one of her paintings in his private study.

"Every time I look at that painting, I think about Grace, and I think about the life that she lived and the life that lay ahead of her, and most of all I think about how when it comes to protecting the most vulnerable among us, we must act now," he said.

As he announced the gun measures, Obama was accompanied by four children chosen from among those who sent letters to him about gun violence and school safety. "We should learn from what happened at Sandy Hook. I feel really bad," 8-year-old Grant Fritz wrote, in a portion Obama read from the podium.

The most contentious piece of the package is Obama's call for a renewed ban on military-style assault weapons, a move that is unlikely to win approval because Republicans who control the House of Representatives are expected to oppose it.

The Newtown gunman, 20-year-old Adam Lanza, used a Bushmaster AR-15 type assault rifle to shoot his victims, most of them 6- and 7-year-olds, before killing himself.

Law enforcement experts have noted, however, that the tighter background checks that Obama is proposing would not have prevented the Connecticut school massacre because the gunman's weapon was purchased legally by his mother.

New York Mayor Michael Bloomberg, a staunch gun control advocate, said tighter controls were needed no matter what.

"No piece of legislation is perfect and no piece of legislation is 100 percent effective. Think of it like a speeding limit. You may every once in a while violate the speeding limit, but that doesn't mean we shouldn't have speeding limits - they protect people's lives," he told reporters.

At the firearm industry's largest trade show in Las Vegas, Gary Svecko - adding a Glock 17 pistol to his guncollection - dismissed Obama's bid to ban assault weapons purchases and blamed video games for inciting violence.

Shares of gun manufacturers, including Smith & Wesson and Sturm Ruger, rose more than 5 percent after Obama unveiled his proposals. Since Newtown, FBI background checks required for gun purchases have soared, indicating more people are trying to buy weapons, likely out of concern that new restrictions may be imposed.

Controversial NRA ad campaign

Underscoring the tough political battle ahead, the NRA launched an advertising campaign against Obama's gun control effort and deployed its lobbyists in force on Capitol Hill.

The NRA, in a TV and Internet spot, accused Obama of being "just another elitist hypocrite" for accepting Secret Service protection for his young daughters but turning down the lobby group's proposal to put armed guards in all schools.

"Only honest, law-abiding gun owners will be affected and our children will remain vulnerable to the inevitability of more tragedy," the group said in response to Obama's proposals.

Administration officials sketched out legislative goals in a conference call with reporters but offered no draft legislation or any clear explanation of how they would overcome the hurdles. They said the list of executive actions would cost $500 million in the federal budget for the 2014 fiscal year.

With gun ownership rights enshrined in the Second Amendment of the U.S. Constitution, gun restrictions have long been a divisive - and risky - issue in American politics.

But polls show public sentiment shifted in favor of tighter gun control fueled by outrage after Newtown, and Obama hopes to take advantage while there is a mood for action in Washington. The pattern after shooting tragedies is that memories of the events soon fade, making it hard to sustain a push for policy changes.

Obama acknowledged the political challenges but made clear he is prepared to take on the NRA, despite its support among Republicans and significant backing among Democrats.

He warned that opponents of his effort would try to "gin up fear" and urged lawmakers to put children's safety above getting an 'A' grade from the gun lobby that supports their campaign."

Michael Steel, a spokesman for Republican House of Representatives Speaker John Boehner, was noncommittal. "House committees of jurisdiction will review these recommendations. And if the Senate passes a bill, we will also take a look at that," he said.

Senate Majority Leader Harry Reid, a pro-gun-rights Democrat from Nevada, also responded cautiously, saying "all options should be on the table" to reduce gun violence.

Obama's initiative treads carefully on whether violent movies and video games contribute to gun violence. An administration official said, however, that Obama would seek $10 million to fund studies of the causes of gun violence, including any relationship to video games and media images.

Wednesday's proposals stem from a monthlong review led by Vice President Joe Biden, who met advocates on both sides, including officials from the arms and entertainment industries.